Magnetic resonance method and device

ABSTRACT

The invention relates to a method for magnetic resonance imaging (MRI) of at least a portion of a body placed in a stationary and substantially homogeneous main magnetic field. According to this method, the portion of the body is initially subjected to a T 2 -preparation sequence (T2PRE). Thereafter, a 2D navigator sequence (NAV) is applied and an MR navigator signal is measured. A series of MR imaging signals is subsequently generated by an imaging sequence (TFE). These MR imaging signals are measured for reconstructing an MR image therefrom. In order to provide a MRI method for T 2 -weighted imaging, which gives a high T 2  contrast and also guarantees a faultless functioning of the navigator, the invention proposes to apply a  2 D navigator restore sequence (NAVRE) prior to irradiation of the  2 D navigator sequence (NAV).

The invention relates to a method for magnetic resonance imaging of atleast a portion of a body placed in a stationary and substantiallyhomogeneous main magnetic field, the method comprising the followingsteps:

a) subjecting said portion to a T₂-preparation sequence;

b) further subjecting said portion to a 2D navigator sequence;

c) measuring a MR navigator signal;

d) generating a series of MR imaging signals by subjecting said portionto an imaging sequence;

e) measuring said MR imaging signals for reconstructing an MR image fromsaid signals.

Furthermore, the invention relates to a device for magnetic resonanceimaging for carrying out this method.

In magnetic resonance imaging (MRI), pulse sequences consisting of RFand magnetic field gradient pulses are applied to an object (a patient)to generate magnetic resonance signals, which are scanned in order toobtain information therefrom and to reconstruct images of the object.Since its initial development, the number of clinical relevant fields ofapplication of MRI has grown enormously. MRI can be applied to almostevery part of the body, and it can be used to obtain information about anumber of important functions of the human body. The pulse sequencewhich is applied during a MRI scan determines completely thecharacteristics of the reconstructed images, such as location andorientation in the object, dimensions, resolution, signal-to-noiseratio, contrast, sensitivity for movements, etcetera. An operator of aMRI device has to choose the appropriate sequence and has to adjust andoptimize its parameters for the respective application.

Known methods of the type specified above can be employed for magneticresonance angiography (MRA), particularly for coronary MRA.

T₂-weighted imaging sequences are used in general clinical applicationbecause they provide exquisite soft tissue contrast. Known imagingmethods consist of an initial contrast preparation period, during whichthe longitudinal magnetization is prepared according to the desiredcontrast. Such a T₂-preparation sequence enables the production ofT₂-weighted images. This is particularly useful in coronary MRA, becausean enhanced contrast between the blood in the coronary arteries and themyocardium is obtained (so called bright-blood methods).

Because respiratory motion of the heart can severely deteriorate theimage quality of cardiac MR imaging, gating and image correction basedon MR navigator signals was introduced to reduce these artifacts. Bymeans of such MR navigator signals, the position of the diaphragm can bemonitored and used as an input for an appropriate gating algorithm.Furthermore, the information of the navigator signal may be used toperform motion correction to improve image quality.

For registering the MR navigator signals, so-called 2D RF pulses may beused. These excite a spatially restricted volume, for example of pencilbeam shape, which is read out using a gradient echo. This allows tomonitor motions of the examined portion of the body along one direction.In coronary MRA, the navigator volume is usually localized at the domeof the right hemidiaphragm such that the motion of the diaphragm can beobserved by the image contrast between the liver and the lung.

The above-mentioned 2D RF pulses consist of shaped RF pulses which areirradiated in combination with fast magnetic field gradient switching.It has been shown, that this technique facilitates the excitation ofarbitrarily shaped profiles in two dimensions.

Subsequent to the T₂-preparation and the measurement of the MR navigatorsignals, usually a series of phase-encoded spin echoes is generated byan appropriate imaging sequence of RF pulses and magnetic field gradientpulses. These spin echoes are measured as MR imaging signals forreconstructing an MR image therefrom, for example by 2D Fouriertransformation.

A T₂-contrast enhanced MRA procedure of the type specified above is forexample described in a publication by Botnar (R. M. Botnar et al., “AFast 3D Approach for Coronary MRA”, Journal of Magnetic ResonanceImaging, volume 10, pages 821-825, 1999). According to this knownmethod, at first a T₂-preparation sequence is applied in order to obtainthe desired contrast between blood and muscle. Thereafter, the patientis subjected to a so-called regional saturation pulse for suppression ofsignal contributions from the chest wall. The next step is theapplication of the 2D navigator sequence and the measurement of the MRnavigator signal. According to the above article, then a spectralsaturation inversion recovery sequence in employed for fat suppressionprior to the actual imaging sequence, which is a so-called 3D TFE-EPIsequence.

With this known method, it is advantageous that the navigator signalcomes immediately before the imaging sequence. It has been shown bySpuentrup (Spuentrup et al., “The Impact of Navigator Timing Parametersand Navigator Spatial Resolution on 3D Coronary Magnetic ResonanceAngiography”, Journal of Magnetic Resonance Imaging, volume 14, pages311-318, 2001) that it is crucial to minimize the delay between thenavigator and the imaging sequence. But one of the main drawbacks of theknown method is that the initial T₂-preparation disturbes the generationand registration of the MR navigator signal. This is because thelongitudinal magnetization of the lung-liver interface, which is used tomonitor the position of the diaphragm during the respiratory motion ofthe patient, is substantially reduced due to the precedingT₂-preparation sequence. As a result, the navigator may fail to detectthe diaphragmatic position such that a diagnostic image of sufficientquality can not be generated. This is particularly valid if, dependingon the position of the navigator, structures within the liver such asthe gall bladder, which has a long T₂, produce bright navigator signals.These signals might easily be misinterpreted by the involved algorithms.

Therefore, it is readily appreciated that there is a need for an MRImethod which enables T₂-contrast enhanced imaging without limiting thequality of the MR navigator signal. It is consequently the primaryobject of the present invention to provide a method for T₂-weightedimaging, which gives a high T₂ contrast and also guarantees a faultlessfunctioning of the navigator.

In accordance with the present invention, a method for magneticresonance imaging of the type specified above is disclosed, wherein theaforementioned object is achieved by subjecting the portion of the bodyto a 2D navigator restore sequence prior to subjecting the portion tothe 2D navigator sequence in step b).

The present invention enables to perform fast tomographic scanning withenhanced T₂ contrast. While the method of the invention is particularlyvaluable for MRA, it can also be applied to any navigator based imagingtechnique. The structure of the imaging procedure is similar to theabove-described known method. But the essential difference is theapplication of the 2D navigator restore sequence, which is generatedprior to the actual 2D navigator sequence. The 2D navigator restoresequence of the invention comprises RF pulses and magnetic fieldgradient pulses, which are selected such that the effect theT₂-preparation sequence has on the MR navigator signal is largelycompensated for. This compensation can effectively be performed, becausewith the 2D navigator restore sequence it is possible to selectivelymanipulate nuclear magnetization in the particular restricted volume,which is subsequently sampled by the 2D navigator sequence in theabove-described known fashion.

The application of a 2D navigator restore sequence is known in adifferent context from a publication by Stuber (M. Stuber et al.,“Three-Dimensional High-Resolution Fast Spin-Echo Coronary MagneticResonance Angiography”, Magnetic Resonance in Medicine, volume 45, pages206-211, 2001). But in contrast to the present invention, this knownpublication is dealing with the so-calles black blood technique, inwhich an initial RF pulse for non-selective inversion of the nuclearmagnetization is followed by a selective inversion pulse forre-inversion of the magnetization. After the initial pulse, there is aninversion delay to facilitate signal-nulling of the in-flowing blood atthe region of interest. According to the above publication, a 2Dnavigator restore sequence is implemented, which locally reinverts (i.e.restores) the longitudinal magnetization at the position of thenavigator. This known method does obviously not enable T₂-weightedimaging with a high T₂ contrast and with a well-functioning navigator,as it is the object of the present invention.

With the method of the present invention it is practical if theT₂-preparation sequence comprises at least two RF pulses, which areseparated by a relaxation period, for enhancing the contrast betweentissues with different transverse relaxation times. With the initialRF-pulse, which is preferably a 90° pulse, the equillibriummagnetization is transformed into transverse magnetization. Onlymagnetization of tissue with a long T₂ will survive the subsequentrelaxation period. After the relaxation period, the remaining transversemagnetization is transformed back into longitudinal magnetization by theso-called “tip-up” RF pulse of the T₂-preparation sequence, which againpreferably has a flip angle of 90°. It is also possible that theT₂-preparation sequence further comprises an even number ofsubstantially 180° RF pulses, thereby avoiding preliminary loss oftransverse magnetization because of local inhomogeneities of the mainmagnetic field.

A 2D navigator sequence, which comprises at least two shaped RF pulsesand at least one gradient pulse being switched during irradiation ofsaid shaped RF pulse, is well suited for application according to themethod of the invention in order to enable the excitation of nuclearmagnetization within a spatially restricted navigator volume. In thisway, the 2D navigator restore sequence can be applied during therelaxation period of the T₂-preparation sequence for selectivelytransforming transverse magnetization within the navigator volume intolongitudinal magnetization. This procedures enables the simultaneousapplication of the T₂-preparation sequence and the 2D navigatorsequence, which is particularly advantageous regarding the speed of theimaging procedure. No additional time is needed for the 2D navigatorrestore sequence by integrating it into the T₂-preparation sequence. Inpractice, the transverse magnetization, which is generated by theinitial RF pulse of the T₂-preparation sequence, is immediatelytransformed back into longitudinal magnetization by the 2D navigatorrestore sequence. At the end of the relaxation period, this longitudinalmagnetization is again transformed into transverse magnetization suchthat it can be restored into longitudinal magnetization by thenon-selective tip-up pulse of the T₂-preparation sequence.

In practice, the MR navigator signal of the present invention canadvantageously be employed for gating of the imaging sequence and/or foradjusting the parameters of said imaging sequence and/or for correctionof said MR image. Regarding the image quality, good results are obtainedif both gating and adaptive motion correction of the imaged volume(so-called slice-tracking) are performed.

In terms of imaging speed, it is particularly useful if the imagingsequence of the method of the invention is a turbo field echo (TFE)sequence. It turns out in practice that good results are obtained with a3D TFE-EPI sequence with partial k-space acquisition.

It is easily possible to incorporate the method of the present inventionin a dedicated device for magnetic resonance imaging of a body placed ina stationary and substantially homogeneous main magnetic field. Such aMRI scanner comprises means for establishing the main magnetic field,means for generating gradient magnetic fields superimposed upon the mainmagnetic field, means for radiating RF pulses towards the body, controlmeans for controlling the generation of the gradient magnetic fields andthe RF pulses, means for receiving and sampling magnetic resonancesignals generated by sequences of RF pulses and switched gradientmagnetic fields, and reconstruction means for forming an image from saidsignal samples. In accordance with the invention, the control means,which is usually a microcomputer with a memory and a program control,comprises a programming with a description of an imaging procedureaccording to the above-described method of the invention. For ECG-gatingof the imaging procedure, ECG-means may be provided for registeringECG-data from the body of the patient. These ECG-data may be processedby the control means of the MRI scanner.

The invention further relates to a computer programme as defined inclaim 10. When loaded in the computer of the MR-system enables theMR-system to perform the method of the invention. The computer programmeaccording to the invention enables the magnetic resonance imaging systemto achieve the technical effects involved in performing the magneticresonance imaging method of the invention. The computer programme isloaded in the computer of micro-processor of the magnetic resonanceimaging system. The computer programme of the invention may be providedon a data carrier such as a CD-ROM or may be made available via a datanetwork, such as the world-wide web.

The following drawings disclose preferred embodiments of the presentinvention. It should be understood, however, that the drawings aredesigned for the purpose of illustration only and not as a definition ofthe limits of the invention.

In the drawings

FIG. 1 shows a diagram of a pulse sequence in accordance with thepresent invention;

FIG. 2 shows an embodiment of a MRI scanner according to the invention.

A sequence design in accordance with the method of the present inventionis depicted in FIG. 1. The diagram shows the temporal succession ofradio frequency pulses RF and of magnetic field gradient pulses GX, GY,GZ in three orthogonal directions. A patient placed in a stationary andsubstantially homogeneous main magnetic field is subjected to thesepulses during the MRI procedure of the invention.

The sequence begins with a T₂-preparation sequence T₂PRE comprising twonon-selective RF pulses α_(X), which are separated by a relaxationperiod. Only nuclear magnetization of tissue with a long T₂ will survivethe relaxation period of the sequence T2PRE. This magnetization istransformed into longitudinal magnetization with the second α_(X) pulse.The T₂-preparation sequence T2PRE further comprises two 180° _(Y) RFpulses in order to avoid preliminary loss of transverse magnetizationbecause of local inhomogeneities of the main magnetic field.

As further shown in FIG. 1, a 2D navigator sequence NAV is applied afterthe sequence T2PRE. The sequence NAV comprises a 2D pulse consisting ofa shaped RF pulse, during which gradients GX and GY are switchedrapidly. A restricted two-dimensional spatial profile, as for example apencil beam shaped navigator volume at the dome of the right diaphragmof the patient, is excited by these pulses. At the end of the 2Dnavigator sequence NAV a MR navigator signal is measured in the presenceof a gradient GZ, thereby enabling the reconstruction of aone-dimensional image of the navigator volume. This image can be used tomonitor the position of the patient's diaphragm during respiration.

A series of MR imaging signals is generated by subjecting the patient toa turbo field echo sequence TFE. These signals are measured and used forreconstruction of an diagnostic MR image, for example of the coronaryarteries of the patient. The navigator signals, which had been measuredduring the sequence NAV, are used for gating of the imaging sequence TFEand for correction of the reconstructed MR image.

In accordance with the invention, a 2D navigator restore sequence NAVREis applied prior to the 2D navigator sequence NAV. In FIG. 1, thesequence NAVRE is incorporated into the sequence T2PRE in order not toloose any time with the application of additional pulses. The 2Dnavigator restore sequence NAVRE comprises a first 2D pulse, which isirradiated immediately after the first α_(X) pulse, thereby selectivelytransforming the transverse magnetization of the navigator volume, whichwas generated by the initial RF pulse ax, back into longitudinalmagnetization. This longitudinal magnetization is not affected bytransverse relaxation during the relaxation period. A second 2D pulse ofthe sequence NAVRE is applied just before the second α_(X) pulse. Thelongitudinal magnetization of the navigator volume is again transformedinto transverse magnetization such that it is restored into longitudinalmagnetization by the second “tip-up” α_(X) pulse of the T₂-preparationsequence T2PRE. As a result, the nuclear magnetization of the navigatorvolume is virtually not disturbed by the T₂-contrast enhancing sequenceT2PRE.

In FIG. 2 a magnetic resonance imaging device 1 is diagrammaticallyshown. The apparatus 1 comprises a set of main magnetic coils 2 forgenerating a stationary and homogeneous main magnetic field and threesets of gradient coils 3, 4 and 5 for superimposing additional magneticfields with controllable strength and having a gradient in a selecteddirection. Conventionally, the direction of the main magnetic field islabelled the z-direction, the two directions perpendicular thereto thex- and y-directions. The gradient coils are energized via a power supply11. The apparatus 1 further comprises a radiation emitter 6, an antennaor coil, for emitting radio frequency (RF) pulses to a body 7, theradiation emitter 6 being coupled to a modulator 8 for generating andmodulating the RF pulses. Also provided is a receiver for receiving theMR-signals, the receiver can be identical to the emitter 6 or beseparate. If the emitter and receiver are physically the same antenna orcoil as shown in FIG. 2, a send-receive switch 9 is arranged to separatethe received signals from the pulses to be emitted. The receivedMR-signals are input to a demodulator 10. The modulator 8, the emitter 6and the power supply 11 for the gradient coils 3, 4 and 5 are controlledby a control system 12 to generate the above-described sequence of RFpulses and a corresponding sequence of magnetic field gradient pulses.The control system is usually a microcomputer with a memory and aprogram control. For the practical implementation of the invention itcomprises a programming with a description of an imaging procedureaccording to the above-described method. The demodulator 10 is coupledto a data processing unit 14, for example a computer, for transformationof the received echo signals into an image that can be made visible, forexample on a visual display unit 15. There is an input means 16, e.g. anappropriate keyboard, connected to the control system 12, which enablesan operator of the device to interactively adjust the parameters of theimaging procedure.

1. Method for magnetic resonance imaging of at least a portion of a bodyplaced in a stationary and substantially homogeneous main magneticfield, the method comprising the following steps: a) subjecting saidportion to a T₂-preparation sequence; b) further subjecting said portionto a 2D navigator sequence; c) measuring a MR navigator signal; d)generating a series of MR imaging signals by subjecting said portion toan imaging sequence; e) measuring said MR imaging signals forreconstructing an MR image from said signals; wherein prior tosubjecting said portion to said 2D navigator sequence in step b), saidportion is further subjected to a 2D navigator restore sequence. 2.Method of claim 1, wherein said T₂-preparation sequence comprises atleast two RF pulses (α_(X)), which are separated by a relaxation period,for enhancing the contrast between tissues with different transverserelaxation times.
 3. Method of claim 1, wherein said 2D navigatorsequence comprises at least one shaped RF pulse and at least onegradient pulse being switched during irradiation of said shaped RF pulsein order to excite nuclear magnetization within a spatially restrictednavigator volume.
 4. Method according to claim 2, wherein said 2Dnavigator restore sequence is applied during said relaxation period inorder to selectively transform transverse magnetization within saidnavigator volume into longitudinal magnetization.
 5. Method of claim 2,wherein said T₂-preparation sequence further comprises an even number ofsubstantially 180° RF pulses.
 6. Method of claim 1, wherein said MRnavigator signal is employed for gating of said imaging sequence and/orfor adjusting the parameters of said imaging sequence and/or forcorrection of said MR image.
 7. Method of claim 1, wherein said imagingsequence is a turbo field echo sequence.
 8. Device for magneticresonance imaging of a body placed in a stationary and substantiallyhomogeneous main magnetic field, the device comprising means forestablishing said main magnetic field, means for generating magneticfield gradients superimposed upon said main magnetic field, means forradiating RF pulses towards said body, control means for controlling thegeneration of said magnetic field gradients and said RF pulses, meansfor receiving and sampling magnetic resonance signals generated bysequences of RF pulses and switched magnetic field gradients, andreconstruction means for forming an image from said signal samples,wherein said control means comprises a programming with a description ofan imaging procedure according to the method of claim
 1. 9. Device ofclaim 8, wherein it comprises ECG-means for registering ECG-data fromsaid body, said ECG-data being processed by said control means forgating said imaging procedure.
 10. A computer readable medium containinginstructions for controlling a computer system: a) subject a portion ofan object to be examined to a T₂-preparation sequence; b) furthersubject said portion to a 2D navigator sequence; c) measure a MRnavigator signal; d) generate a series of MR imaging signals bysubjecting said portion to an imaging sequence; e) measure said MRimaging signals for reconstructing an MR image from said signals;wherein the computer program further has instructions to prior tosubjecting said portion to said 2D navigator sequence in step b),subject said portion is further to a 2D navigator restore sequence.